Explanation:

To determine which elements produce H2 gas with NaOH, we need to understand the reaction between these elements and NaOH.

Reaction:
The reaction between a metal and NaOH can be represented as follows:
2M + 2NaOH → 2NaM + H2

Here, M represents the metal.

Analysis of Elements:
Let's analyze each given element and determine if it will produce H2 gas with NaOH.

1. Zn (Zinc):
Zinc reacts with NaOH to produce sodium zincate and hydrogen gas.
Zn + 2NaOH → Na2ZnO2 + H2

2. Al (Aluminum):
Aluminum reacts with NaOH to produce sodium aluminate and hydrogen gas.
2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2

3. Sn (Tin):
Tin does not react with NaOH to produce hydrogen gas. Therefore, it does not contribute to the total number of elements that produce H2 gas with NaOH.

4. Pb (Lead):
Lead does not react with NaOH to produce hydrogen gas. Therefore, it does not contribute to the total number of elements that produce H2 gas with NaOH.

5. P (Phosphorus):
Phosphorus does not react with NaOH to produce hydrogen gas. Therefore, it does not contribute to the total number of elements that produce H2 gas with NaOH.

6. S (Sulfur):
Sulfur does not react with NaOH to produce hydrogen gas. Therefore, it does not contribute to the total number of elements that produce H2 gas with NaOH.

Conclusion:
Based on the analysis, we can determine that out of the given elements, only Zinc (Zn), Aluminum (Al), Tin (Sn), and Lead (Pb) can produce hydrogen gas with NaOH. Therefore, the correct answer is '4'.